Muzzle velocities of bullets in small arms ammunition have been increasing steadily since the adoption of smokeless powder as a propellant. All lead bullets, fired at high velocity, will slip relative to the barrel or rifle, and consequently will not be caused to spin sufficiently, thereby causing unstable flight. It also results in unacceptable fouling of the bore of the rifle. With the higher velocities came the requirement to encase the lead cores of the rifle bullets with a relatively hard jacket.
Over the years, the development of faster and flatter shooting cartridges has caused the rifle user to desire a bullet which will perform well at longer and longer ranges while still performing adequately at short ranges. The desired window of operating performance has been widening at both the high end and the low end velocities. In order for the bullet to perform well at these two velocity extremes, several things must occur.
At the low velocity end of the window, the bullet must be sufficiently weakened to initiate a low velocity expansion. A number of patents have issued with various concepts as to how to accomplish such expansion and such velocities. Some of the most notable and commonly used methods today include that of the Whipple U.S. Pat. No. 2,327,950, which utilizes a tapered wall thickness which is progressively thinned toward the mouth of the jacket, along with a top edge which is scalloped. A somewhat similar concept is disclosed and utilized in the Burns & Schreiber U.S. Pat. No. 3,143,966. This patent teaches that the tapering mouth of the jacket should be folded or "inwardly-pinch pleated". Another of the most common methods is that taught by Schreiber U.S. Pat. No. 2,838,000 in which longitudinally extending troughs, or thinned areas of the jacket, extend rearwardly from the mouth of the jacket. Each of these methods has been somewhat successful, in various degrees, in initiating low-velocity expansion of the bullet.
The method of the French U.S. Pat. No. 2,765,738 is interesting, although it is not commonly used. The inventor uses a process in which the mouth of the jacket is formed with longitudinally extending, alternating thick and thin sections, such as those formed by a faceted punch. The jacket is then annealed, and finally, drawn an additional time to a uniform wall thickness. The result of this process is a jacket with longitudinally extending hard and soft sections at the mouth of the jacket.
At the other end of the window, bullets must be sufficiently strong to withstand the extremely high stress load imposed upon it by high velocity impacts. This requires some feature which will slow down and stop the expansion process before the bullet over-expands and fragments. In most conventional bullets, this feature is provided to a limited extent by thickening the jacket wall. This method works reasonably well; however, it has a significant drawback in that it has a fairly small performance operating window. To understand the reason for this drawback, one must look at the methods used for manufacturing most conventional jackets.
The method most commonly utilized is one in which a cup is formed from a flat sheet of copper or brass. This cup has a relatively thick wall and is usually softened prior to subsequent forming operations by annealing same. The cup is then run through a draw operation which reduces the wall thickness and correspondingly lengthens the jacket. This drawing process, however, work-hardens the jacket material.
Some of such jackets may have the final desired wall thickness and mouth taper after a single draw operation. Others may require two or more draws with an intermediate annealing step. In either case, however, the final jacket is relatively hard, usually with the mouth portion being harder than the shank portion. This is due to the required mouth-tapering operation which work-hardens the mouth much more than the thicker-walled shank because the metal is moved to a greater extent.
A bullet made from a jacket having a mouth harder than its shank limits the performance operating window of the bullet. Thus, if the bullet made by the above practices is sufficiently soft or weak to initiate expansion at low velocity, it is then too soft or weak to withstand the high loads imposed thereupon at high velocity impacts, and the bullet will then fragment or disintegrate. Conversely, if the bullet is made in accordance with the above procedures, and is sufficiently strong to withstand the high velocity loads, then it is too hard and strong to initiate expansion at low velocity. Bullets of this type typically have an operating window of about 600-700 FPS. The advantage of this type of bullet is that it is relatively inexpensive to manufacture. Bullet manufacturers have, for the most part, set the operating window requirements to be between about 2,000 FPS and 2,700 FPS. Below 2,000 FPS, the bullets so manufactured do not expand; and above 2,700 FPS, significant fragmentation occurs. Thus, it is apparent that there is a significant need for bullets having a substantially widened performance operating window.
Some bullet manufacturers have been successful in overcoming this problem by designing bullets which are relatively expensive to manufacture. Most notably, the Trophy Bonded Bearclaw bullet uses a jacket having a soft mouth due to the bonding process used in the manufacture of the bullet, and yet has a solid shank which stops the expansion. Another concept which is utilized is the Nosler Partition, which has a center rib that stops the expansion. The disadvantage of these designs, however, is the substantially higher manufacturing cost.
Present day bullet needs continue to increase. Specifically, hunters have increasingly demanded bullets of improved performance. Thus, the desired ranges have steadily increased, as have the performances for a given range. As a consequence, there is a need for a rifle bullet of any given caliber which will perform adequately over an ever-expanding distance range.
The mushrooming bullet has been designed to provide marked increase in shock. Problems with respect thereto have risen, however, with respect to fragmentation, with attendant reduction in shock, damage to game meat, etc. Ideally, the mushrooming bullet would remain intact throughout, but such has not been the case. For high velocities, means for slowing down and eventually stopping the expansion process, before the bullet over-expands and fragments, is needed. This function is provided by the relatively hard thick shank of my bullet jacket.
Penetration is also a desirable feature in a bullet, but it is improved only by retaining its initial shape, which is the antithesis of mushrooming. One of its most serious problems is the tendency of a bullet's lead core to separate from its metal jacket and , in general, to disintegrate and/or lose momentum when such separation takes place.
In seeking to solve the above problems and others, ammunition manufacturers have made numerous changes from time to time, until today the jacket of a conventional bullet has a mouth portion which is harder than its shank, which limits the performance operating range of the bullet. Since the velocity of a bullet decreases with distance from the muzzle of the rifle, the velocity of the bullet is less when it reaches a long range shot target than a short range shot target.
Variations in the strength of the bullet for expansion are conventionally provided by varying the strength of the jacket adjacent its mouth. Thus, it is common to weaken the mouth portion of the jacket when it is desired to facilitate the initiation of expansion, commonly referred to as "mushrooming."
Today, if the jacket is sufficiently soft or weak adjacent its mouth to initiate expansion at low velocity (such as found at long range shots), it is then too soft or weak to withstand the high forces it is subjected to at high velocity impacts provided by short range shots. As a consequence, such a bullet will fragment or disintegrate under such high forces.
Conversely, if the jacket is made to be sufficiently strong to withstand the high velocity forces (as provided by short range shots), then it is too hard and strong to initiate expansion at low velocity (as provided by long range shots) which results in reduced effectiveness of the bullet. Such a bullet penetrates but has no expansion.
Bullet manufacturers recognize the above problems and seek to solve them by seeking a middle-ground somewhere in between the ideal velocities for short and long range shots. Thus, they have adopted a relatively narrow operating window by producing bullets having a window of only about 2,000 fps-2,700 fps. Such bullets typically will not expand at velocities of less than 2,000 fps. Likewise, such bullets will typically fragmentize significantly at velocities above 2,700 fps. Manufacturers produce and market bullets having such narrow operating windows because they are relatively inexpensive to produce, and because no one has heretofore proposed a correspondingly inexpensive bullet which will meet the requirements for a wider operating window.